This invention pertains generally to the field of accelerators for atomic and nuclear particles, and more particularly, to linear accelerators which utilize radio-frequency quadrupole (RFQ) electric fields for accelerating, focusing, and bunching a beam of ions.
For many years it has been well-known that conventional linear accelerators (linacs) employing drift tubes and the like, with magnetic accelerating and focusing fields, are generally inadequate for transporting and accelerating ion beams at low energies. The main drawback of these conventional linacs is that the particle velocities in such ion beams is so low that the Lorentz forces on the particles are too small to control the beam for any magnetic fields that can be achieved practically. In order to accelerate ions in conventional linear accelerators, one must employ an injection system between the ion source and the accelerator to raise the energy of the beam particles and to focus and bunch these particles to obtain a beam that is suitable for acceleration. For several decades the design of this injection system, i.e. an accelerator for low-energy ion beams, presented a challenge to researchers in this field.
In 1970 I. M. Kapchinskii and V. A. Teplyakov suggested the RFQ linear accelerator as a possible solution to this problem ("Linear Ion Accelerator with Spatially Homogeneous Strong Focusing", Prib. Tekh. Eksp. 2, 19 (1970)). This device contains no drift tubes, but rather comprises four elongated electrodes disposed symmetrically around the beam, each electrode extending in a direction parallel to the beam axis. The electrodes are driven by radio-frequency (rf) electrical power, such that the voltage on each electrode is approximately constant along its entire length at any given time. Furthermore the voltages of each pair of electrodes on opposite sides of the beam axis are the same, and are equal in magnitude and opposite in sign to the voltages on the other pair of oppositely-disposed electrodes, so that all points in the beam the electric fields in the plane perpendicular to the beam axis are primarily quadrupolar. The particle beam is thereby exposed to an alternating-gradient quadrupole electric field which produces the well-known strong-focusing effect, and this effect is independent of the velocity of the beam particles.
Of course, if the electrodes are at a constant distance from the beam axis along their entire length, then the electric fields are completely transverse to this axis. The above authors pointed out, however, that if the distance of each pair of diametrically opposed electrodes from the beam axis varies with spatial periodicity along this axis, and if the distance of the adjacent pair of oppositely-charged electrodes also varies with the same period, but with a phase difference along the axis of 180.degree. relative to the first electrode pair, then an electric field component parallel to the beam axis will be produced. Thus, for each of the electrodes the surface facing toward the beam axis is rippled so that the distance of this surface from the axis oscillates between a minimum value, a, and a maximum value, ma (m&lt;1), as one proceeds in the direction parallel to the beam axis (conventionally defined as the z-direction). The distance, d, between adjacent ripples on a given electrode, and the minimum and maximum distances from the electrode to the beam axis (a and ma) are the same for all four electrodes. For a given pair of electrodes lying in a plane passing through the beam axis, the crests of the ripples occur at the same positions along the beam axis, and these positions also mark the location of the ripple troughs in the other pair of electrodes lying in the orthogonal plane through the beam axis. The electric field extending from the ripple crests of one pair of electrodes to the crests of the adjacent electrodes lying in the orthogonal plane therefore has an axial component.
The ripple crests define the boundaries of a series of unit cells arranged along the beam axis, each cell having a width d/2 in the z-direction. At all points within any given unit cell the z-component of the electric field is in the same direction along the beam axis, and in the adjacent unit cells on either side of the given cell the z-component is in the opposite direction. Therefore the electric fields in successive unit cells have an alternately accelerating and decelerating effect on the beam particles, and these fields also tend to cause the beam to bunch in alternate cells. For a given beam particle velocity, v, the frequency, f, of the electrode voltage oscillations is such that the period of these oscillations equals the transit time of the particles through the distance d, EQU f=v/d,
so that the particle bunches will continue to encounter an accelerating electric field as they move in the z-direction from one unit cell to the next cell.
Therefore, the RFQ linear accelerator structure suggested by Kapchinskii and Teplyakov is capable of focusing, bunching and accelerating a beam of charged particles even for low particle velocities. Of course, as the particles are accelerated in traveling down the length of this structure their velocities will increase. This implies that the distance, d, between ripples on a given electrode surface must be made larger in the downstream portions of the accelerator. The magnitude of the acceleration will be affected by the dimensions of the ripples, a and ma, and by the magnitudes of the electrode voltages; these voltages, however, characterize the whole structure and only determine an overall scale factor for the amount of energy transferred to the beam particles. For example, if the ripple dimensions, a and ma, are constant down the entire length of the electrodes, and if we assume that the axial accelerating fields are constant in time as seen by the beam particles, the acceleration of these beam particles will be constant and the speed of the particles will be proportional to the square root of the distance traveled. (We are assuming also that the beam particle velocities are sufficiently low that the effects of special relativity may be ignored.) This implies that the distance d and the widths of the unit cells must also increase in direct proportion to the square root of the axial distance down the accelerator. Obviously this particular dependence of the quantity d on axial position is sensitive to the assumption of constant particle acceleration, and if the heights of the ripples vary with axial position, then the widths of the unit cells should be varied accordingly so that the transit time through a unit cell remains constant, regardless of the beam particle mass or charge, or the electrode voltage.
Various researchers at different laboratories have carried through the detailed design of the electrode geometry and analysis of the particle beam dynamics for a variety of RFQ linacs designed for a number of different practical applications. The typical RFQ linac employs vane-like or rod-like electrodes having values for the ripple sizes a, and ma, that increase gradually with axial distance downstream. At the injection end of the accelerator the axial fields are zero, and the first few unit cells, called the "radial matcher", are designed to optimize the matching of the dc ion beam in the time-varying fields of the accelerator. This section is followed by the "shaper" section, then the "gentle buncher" which produces more efficient adiabatic bunching and higher beam intensities, and finally the accelerator section. Various profiles for the electrode surfaces in the plane transverse to the beam axis have been studied, including the hyperbolic and wedge shapes originally suggested by Kapchinskii and Teplyakov. The design techniques and operating experiences for different types of RFQ linacs have been carefully reviewed in an article by H. Klein ("Development of the Different RFQ Accelerating Structures and Operating Experience", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, August, 1983), and a summary of the various RFQ linacs in operation, under construction, or in the preliminary design phases has been given by S. O. Schriber ("Present Status of RFQ's", 1985 Particle Accelerator Conference, Vancouver, Canada; May 13-17, 1985; IEEE Transactions on Nuclear Science, Vol. NS-32, No. 5, Page 3134 (1985)).
As pointed out by Klein in the above review article, the RFQ designs to date suffer from the disadvantage that the design parameters are strongly dependent on each other, and any given layout tends toward inflexibility. One usually starts by choosing the ion species to be accelerated, having a certain charge-to-mass ratio, and then proceeds to select an operating frequency. These frequencies may vary by a factor of 10 or more, depending upon the desired application and ion species. Once the operating frequency is chosen, a resonating RFQ structure must be designed that will cause the electrode voltages to oscillate at the chosen frequency. These resonators fall generally into two distinct categories: resonant cavities and resonating LC-structures. Resonant cavities are used at frequencies above 150 MHz, because below this limit the dimensions of the cavities become impractically large. The LC-structures are analogous to dual-conductor transmission lines, and are useful at frequencies below 150 MHz. A hybrid type of structure, known as the split coaxial resonator (SCR), has some of the characteristics of both types of rf-structure, but in practical terms it can only be designed for frequencies between a few Mhz up to about 100 Mhz. This SCR structure is described in U.S. Pat. No. 4,404,495 (Mueller), which discloses an embodiment of this device designed to operate at 13.5 MHz to accelerate very heavy ions having an atomic mass/charge ratio in excess of 100, with beam currents in the milliampere range.
In most of the previous designs for RFQ linear accelerators, then, it is found that once an operating frequency and resonating rf-structure have been chosen, the design is fairly "locked in" to that frequency, and to accelerate beams with a different frequency one must make substantial alterations to the resonating structure. This, in turn, limits the beam characteristics that one can obtain with any given RFQ accelerator. For a particular species of ion with a given charge-to-mass ratio, the input and output energies of the beam are limited to the values that correspond to the fixed operating frequency. Of course, each resonating structure generally requires a certain amount of "tuning", i.e. variation of the physical parameters to adjust the resonant frequency to its desired value. Various techniques have been developed for tuning rf resonators, such as the insertion of a vacuum capacitor or "tuning ball" as disclosed in U.S. Pat. No. 4,494,040 (Moretti). In the case of commonly used designs of RFQ resonators, as pointed out by Klein in the article cited above, tuning of the resonators is generally difficult, partly because of the strong interdependence of the RFQ design parameters. In the context of Klein's remarks, of course, "tuning" refers to variations in the operating frequency over a relatively small range.
In short, the advantages of being able to operate a linear ion accelerator over a wide range of frequencies are that, for any given ion species, one can obtain an accelerated beam of various different energies, and conversely, for a given beam energy one can accelerate ions with various different charge-to-mass ratios. The ability to control these parameters over a large range makes available a variety of important applications and interesting experiments for these accelerators, spanning the fields of atomic and solid state physics, nuclear chemistry, and radiation biology, in addition to their usefulness as injectors for larger machines. These advantages have been recognized by researchers in the field of linear accelerator development. For example, M. Odera has described the design and operating characteristics of a frequency tunable linac in Japan ("Report on Frequency Tunable Linac", Proceedings of the 1984 Linear Accelerator Conference, Seeheim, West Germany; GSI-84-11 Conf., p. 36 (September, 1984)). This is a drift-tube accelerator in which the frequency is varied by a quarter-wave coaxial resonator stub with a "race-track" cross section and a movable shorting device, connected and coupled to the drift-tubes. By moving the shorting device over a distance of approximately 2 meters, the operating frequency of the accelerator can be varied between 17 MHz and 60 MHz, although in practice the maximum operating frequency is 45 MHz, based on other practical considerations. With this accelerator ions from Hydrogen to Gold have been accelerated from energies of 0.6 MeV/amu to 4 MeV/amu.
To accelerate the heaviest ions in the periodic table, it is generally desirable to operate at frequencies down to the few-MHz range. With the Japanese machine described above, already at 17 MHz the tuning structure must be over 6 feet long, and of course this machine does not have the additional advantages of the RFQ design. However, the article by Odera illustrates how much flexibility can be obtained with a machine in which the operating frequency can be varied by a factor of three.
A variable freuency RFQ linear accelerator in Frankfurt, West Germany, has been described by A. Schempp and co-workers ("Status of the Frankfurt Zero-Mode Proton RFQ", 1983 Particle Accelerator Conference, Santa Fe, New Mexico; August, 1983; IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, Page 3536 (1983)). The RFQ structure of this machine includes electrodes that are supported by pairs of radial stems at periodic intervals along the electrodes, each stem comprising a flat strip-like conducting support having a U-shaped end, with the flat surfaces of these stems perpendicular to the beam axis. The beam axis passes between the legs of the "U", each of which is attached to one of the equivalent electrodes on opposite sides of the beam axis. The stem extends from the electrode pair to a common conducting support surface, which forms an electrical ground. The adjacent stem in each pair is similarly connected to the opposite pair of electrodes at a slightly displaced axial position, and the two stems extend downward from the electrodes to the electrical ground surface at an angle relative to each other. Each pair of stems together with the conducting ground surface form a lumped inductance element which may be approximated by a single triangle-shaped loop, where the two stems and the electrically grounded support surface corespond to the sides of the triangle. The resonating structure therefore comprises the electrodes loaded periodically with these inductive support stems.
The incorporation of the electrode supports into the resonant rf-structure as periodic inductive loads is a well-known concept. For example, the "spiral stem RFQ resonator" is a system in which the electrode supports are each a spiral coil around the beam axis, with one end connected to a pair of electrodes and the other end connected to the ground surface. This structure is described by both Klein and Schempp, as well as other authors (e.g. R. H. Stokes at al., "A Spiral-Resonator Radio-Frequency Quadrupole Accelerator Structure", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, p. 3530 (August, 1983)). However, the unique feature of the straight support stems in the Frankfurt machine is that the inductance may be varied by connecting a "shorting bar" to each pair of stems at various positions along the length of the support strips. Each stem has a slotted hole extending lengthwise, and the shorting bar is a flat conducting strip-like member having a similarly slotted hole. The bar can be attached to each stem by bolts which pass through the slots in each stem and the bar, and the slots allow this point of attachment to be adjusted, thereby varying the size of the triangular loop and the resulting inductance. This structure is illustrated in FIG. 3 of the article by Schempp et al. cited above, and it has been claimed that this structure allows one to vary the resonance frequency by a factor of 3 (A. Schempp et al., "Zero-Mode-RFQ Development in Frankfurt", Proceedings of the 1984 Linear Accelerator Conference, Seeheim, West Germany; GSI-84-11 Conf., p. 100 (September, 1984)).
There are some obvious drawbacks to this scheme for achieving variable resonance frequencies. Clearly the machine is not intended to allow the frequency to be varied during normal operation. Adjusting the frequency requires entering the vacuum vessel in which the entire assembly is located, and adjusting each shorting bar individually. In fact, the structure for varying the frequency in the Frankfurt machine is really intended as a tuning system, and the authors mention that this is done by removing the RFQ structure from the tank and aligning and tuning it ona bench outside the tank (A. Schempp et al., Nuclear Instruments and Methods in Physics Research, Vol. B10/11, p. 831 (1985)).
Furthermore, it has been observed that in the rf-structure of the Frankfurt machine, there apparently is non-negligible mutual inductance between different pairs of support stems (R. M. Hutcheon, "A Modeling Study of the Four-Rod RFQ", Proceedings of the 1984 Linear Accelerator Conference, Seeheim, West Germany; GSI-84-11 Conf., p. 94 (September, 1984). The operating frequency and design of the machine is necessarily affected by this magnetic cross-coupling of the support stems. This means, for example, that the design of the resonance structure is affected by the length of the machine, because as the length of the electrodes is increased and more support stems are added, the resonance frequency will change. This has been regarded as a "significant limitation" for RFQ linacs in general, in that it places an upper limit on the feasible length of the machine, and therefore a lower limit on the charge-to-mass ratio of the ions that can be accelerated (L. M. Bollinger, "Present Status and Probable Future Capabilities of Heavy-Ion Linear Accelerators", Proceedings of the 10th International Conference on Cyclotrons and Their Applications, Michigan State University, p. 504 (May, 1984)).
Finally, it will be noted that the variable frequency Frankfurt machine is designed to operate as a proton accelerator at 108 MHz, and Schempp and his co-authors indicate that the only resonators that will enable an RFQ linac to operate in the 10-20 MHz range are the split coaxial resonator and the spiral stem resonator, neither of which is designed for variable frequencies. Clearly these authors did not consider their variable-frequency RFQ design to be feasible in the few-MHz frequency range.